There is currently a strong effort towards the definition of the 5G mobile network requirements. Both in the fronthaul (the link between the digital unit and the radio unit) and the backhaul (the connection towards the core network) optical technologies will play a crucial role. It is commonly understood that the high bandwidth to be delivered to the end users and the related adopted technologies, for example MIMO, beamforming, and C-RAN, will require a big increase in capacity which will be likely to require much more than 10 Gbit/s per wavelength.
In the optical fronthaul/backhaul domain (and in general in the optical access) it is common to realize transmission via a single optical fibre instead of a fibre pair because the infrastructure is not owned by the telecom operator but rather by municipality, consortia or the incumbent operator. Single-fibre solutions are a de-facto requirement to halve the number of manual operations (inspection, cleaning, assembly) required by the field technician and reduce to zero the possibility of mis-connection. The requirement for single-fibre transmission means that it is not possible to adopt the current hardware used for two-fibre based 100G transmission because the transmitted and received signals would be centred at the same optical frequency and would therefore be subject to intolerable penalties due to in-band crosstalk arising from unavoidable multiple reflections.
Current solutions for single-fibre working fronthaul and backhaul solutions rely on the well-known intensity-modulated and directly-detected, IM/DD, approach. IM/DD links have been extremely successful for bit-rates up to 10 Gbit/s and are currently advantageously implemented by means of pluggable optics in the XFP or SFP+ formats. The WDM transmission bandwidth is partitioned by means of band splitters and one band is dedicated to West-East transmission and the other to East-West transmission, as described in “Optical Networks—A practical perspective”, 3rd edition, Ramaswami, Sivarajan and Sasaki, 25 Sep. 2009. Direct-detection receivers are wavelength-agnostic over a wide bandwidth, so it is easy to implement the optical link as two separate lightpaths employing two different frequencies. Smaller formats are likely to be developed within the 5G timeframe, but the bit-rate of IM/DD formats is limited by the chromatic and polarization-mode dispersion of single-mode-fibre.
Another technical solution would be to integrate two separate, independently tunable lasers in the transceiver. However this solution faces two problems: higher cost, but this may be overcome by photonic integration; and power dissipation that limits the volume reduction and is a much more serious constraint.
Another straightforward solution is to use two optical fibres for each fronthaul or backhaul link but of course this is undesirable from a business perspective because it increases the operating expense.
A final solution may be to use tightly packed lower-speed optical channels based on IM/DD with interleaved or, generally, non-overlapping wavelengths. Despite the apparent simplicity, this solution would require very precise optical filters and stable lasers, the propagation would be limited by known fibre impairments and, from a business perspective, it would be a proprietary solution requiring dedicated manufacturing.